Science
How do you separate mixtures?
Quick answer
Mixtures can be separated by using the different properties of their parts. Common methods are sieving, filtering, evaporation, magnets and chromatography.
In one sentence
To separate a mixture, choose a method that uses a difference between its parts, such as size, magnetism or whether one part dissolves.
Explained
The parts of a mixture are not chemically joined, so you can separate them by physical methods. The trick is to find a property that is different, then pick the method that uses it.
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- Sieving separates solids of different sizes. A sieve holds back large pieces and lets small ones through, such as flour from lumps, or stones from sand.
- Filtering separates an insoluble solid from a liquid. Pour sandy water through filter paper in a funnel. The water passes through as the filtrate, and the sand stays as the residue.
- Evaporation gets a dissolved solid out of a solution. Heat or leave salt water in the sun and the water turns to gas, leaving crystals of salt. People still make sea salt this way. See evaporation.
- Magnets pull out magnetic materials, like iron and steel, from non-magnetic ones. This is used in recycling plants to pick steel cans out of rubbish. See magnets.
- Decanting and sedimentation. Leave a muddy mixture to stand, and heavy bits sink to the bottom. You can then pour off the liquid on top.
- Distillation. Boil a solution and cool the steam in a separate tube to collect the pure liquid. This is used to get fresh water from sea water.
- Chromatography. Dip a strip of paper with an ink spot into water. The water creeps up the paper and carries the colours at different speeds, so a black ink may split into several colours.
Often you need more than one step. To get clean salt from a mixture of salt, sand and iron filings, you could use a magnet to remove the iron, add water, filter off the sand, and then evaporate the salt water to get the salt. See also mixtures and solutions and materials.
Examples
| Mixture | Method |
|---|---|
| Stones and sand | sieving |
| Sand and water | filtering |
| Salt and water | evaporation |
| Iron filings and sand | magnet |
| Colours in ink | chromatography |
Common mistakes
- Using filtering for a solution. Dissolved salt passes through the paper. Use evaporation.
- Expecting a magnet to pull everything. Only a few metals, mainly iron, are magnetic.
- Heating the solution until it spits. Take care, and ask an adult to help with heat.
Quick quiz
Separating mixtures
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How could you separate sand from water?
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Related topics
Mixtures and solutions
A mixture is two or more substances mixed but not joined chemically. A solution is a mixture where one substance, like salt, dissolves completely in a liquid, like water.
Evaporation
Evaporation is when a liquid slowly turns into a gas from its surface, like a puddle drying in the sun. It happens faster when it is warm, windy or the liquid is spread out.
Materials
Materials are what objects are made from, such as wood, metal, plastic and glass. Each has properties, like being hard, bendy, waterproof or see-through, that suit it to some jobs.
Adaptation
An adaptation is a feature or behaviour that helps a living thing survive where it lives. A polar bear's thick fur and a cactus's spines are adaptations.
Amphibians
Amphibians are cold-blooded animals with thin, moist skin that live part of their lives in water and part on land. Frogs, toads, newts and salamanders are amphibians.
Birds
Birds are warm-blooded animals with feathers, wings and a beak. They lay hard-shelled eggs, and most can fly. Penguins, ostriches and parrots are all birds.